Circuit breakers have been traditionally implemented using electromagnetic components to break a current path in response to an overload current. While these devices are necessary and useful in many applications, there have been substantial efforts to replace them, at least in part, with solid state technology. For example, U.S. Pat. Nos. 4,631,621, 4,626,907, and 4,700,256 each by Howell, disclose solid state circuitry in combination with mechanically switched contacts to address problems relating to voltage arcs between the mechanically switched contacts. This arc problem occurs when the mechanical contacts separate, i.e., when the current path is interrupted, and is known to cause contact and other component related problems.
Solid state circuit breakers that provide a conductive path between the line and the load without mechanically switched contacts have also been described. For instance, in U.S. Pat. No. 4,713,720, Rogers et al., a solid state AC circuit breaker includes a power transistor in a current path between a source and a load. The current path is monitored to determine when an overload current is present, at which time the power transistor is turned off to break the current path.
This type of circuit breaker poses a problem should the solid state switch fail. That is, when a device fails, it predominantly reverts to a low impedance state or "shorted" condition whether it is initially in either the "on" or "off" state.
If failure occurs when the device is conducting current into a load, it will not interrupt the current on command and, hence, loads such as motors may not be turned off at critical times which may cause damage to associated equipment. If failure occurs when the device is clearing a fault, the circuit breaker will have lost its protective function.
A third case arises in which the switch may fail shorted when it has been turned off. Power is then applied inadvertently to a load which may, for example, cause motors to start.
Another problem associated with solid state interrupters is the relatively high cost of the solid state switches and the associated electronic control circuits.
Solid state interrupters also generate a significant amount of heat. Conventional circuit board panels become excessively hot if used to house these devices.
These problems have posed a serious obstacle to the commercial viability of solid state interrupters. They have typically been limited to very special applications such as military or small DC power supplies.
Accordingly, there is a need for a solid state circuit breaker that overcomes these problems.